Light cycle oil (LCO), which is a catalytically cracked gas oil produced by fluid catalytic cracking (FCC), contains large amounts of unstable olefins, and is therefore unsuitable as a gas oil fraction. Accordingly, there are limits to the potential uses of light cycle oil, although various attempts have been made to develop processes for effectively utilizing such light cycle oil.
For example, a process has been disclosed in which a blended oil containing a straight-run gas oil and a light cycle oil produced by FCC is used as a feedstock oil, and this feedstock oil is subjected to hydrodesulfurization in a gas oil desulfurization process using a hydrodesulfurization catalyst (hereinafter also referred to as a “desulfurization catalyst”) (for example, see Patent Document 1).
On the other hand, in the case of gas oils used in diesel fuel and the like, the need to reduce environmental impact has seen a stepwise trend towards low-sulfur (or sulfur-free) oils with a reduced sulfur content. Conventionally, the regulated limit for the sulfur content has been 2,000 ppm by mass, and the sulfur content of the obtained gas oil fraction readily satisfies this prescribed limit even if a light cycle oil is used in the feedstock oil, since the sulfur content in the light cycle oil is 2,000 ppm by mass or less.
However, in recent years, the prescribed limit for the sulfur content in gas oil has been reduced to 10 ppm by mass, and therefore in order to enable a light cycle oil to be used in the above-mentioned feedstock oil, a higher level of hydrodesulfurization must be performed. One possible method for improving the sulfur content reduction effect of a hydrodesulfurization reaction using conventional technology would involve increasing the temperature of the hydrodesulfurization reaction. However, as the reaction temperature is increased, the rate of deactivation in the catalytic activity of the desulfurization catalyst increases markedly, resulting in a significant shortening in the life of the catalyst. Moreover, methods that involve increasing the reaction temperature tend to be accompanied by a deterioration in the color index of the obtained gas oil, making it very difficult to satisfy Japan's strict regulations (L1.5) relating to the color index of gas oils.
Accordingly, obtaining a gas oil fraction that exhibits favorable product properties such as sulfur content and color index using a method that involves increasing the reaction temperature of the hydrodesulfurization reaction is impossible without a significant accompanying reduction in the life of the desulfurization catalyst.
Further, some gas oil products are prepared by blending a gas oil fraction and a kerosene fraction, and in these cases it is important, from the viewpoint of the product combustibility, that the cetane index is a sufficiently high value.